In a motor drive unit for performing film winding operation of a camera, there is a direct-current motor drivingly connected to the film wind-up mechanism in the camera, which is supplied with electrical energy from a battery. Because of the limitation of the capacity of the battery and the power output of the motor and since the motor drive unit is required to be constructed in the form of small size and light weight, it is desired to operate the motor as efficiently as possible. Such operation will also prolong the life time of the battery. A further requirement for the motor drive unit is to allow for increase in the film frame frequency. In this respect, the operating condition of the motor is necessarily at the maximum possible output, particularly when the given photographic situation is at a normal temperature condition.
In general, however, as is understood from the characteristic curves of a D.C. motor, taking the example illustrated in FIG. 1, a value of torque A at which the efficiency reaches the maximum level differs from that of torque B at which the power output reaches the maximum. It is, therefore, well known to those skilled in the art of motor drive units adapted for use with cameras, that the requirement of increasing the winding speed is better fulfilled by operating the motor at the point B for the maximum power output, while the requirement of minimizing the electrical energy consumption to achieve an increase in the number of film cartridges fed per battery is better fulfilled by operating the motor at the point A for maximum efficiency. On the other hand, the relationship between the output torque of the motor and the winding-up torque may be expressed as: ##EQU1## Increase of the winding speed therefore makes the torque of the motor increase. For this purpose, the gear ratio may be decreased. Increase of the number of film cartridges fed per battery decreases the torque of the motor. This calls for an increase in the gear ratio.
The motor drive unit for use with the still camera stands in need of an improvement for snap-shot photography by speeding up the winding operation to as fast a value as possible. On this account, the gear ratio must be decreased so as to effect an approach to the B point. But, when the camera is used under low temperature conditions, since the film is so hardened and the lubricant oil used in the winding mechanism is so increased in viscosity, the winding torque is necessarily increased. This leads to increase of the output torque of the motor provided that the gear ratio remains unchanged. Thus, the consumption of electrical current is increased. Since, as the electrical power source of the motor drive unit, general use is made of a dry battery which deteriorates in performance as the ambient temperature decreases, the adjustment of the gear ratio to the neighborhood of the point B in the characteristic curves of FIG. 1 will result in an extremely large decrease in the number of film cartridges fed per battery due to the increase of the electrical power consumed and the deterioration of the battery.
Besides this, the motor drive unit for the still camera has found its use in a wide range of fields, so there is a demand for making variable the interval between the successive exposures. Consequently, the wider the range of variation of interval, the greater versatility the unit will have.
The conventional motor drive unit for the still camera is designed to have a gear ratio determined by taking into account an intermediate point between the points A and B in the graph for the operating characteristics of the motor, so that the winding speed at the normal temperature can be increased to some extent and the number of film cartridges fed at low temperature can also be increased to some extent. As a result, therefore, it is impossible to operate the motor under the maximum conditions of efficiency and power output.
The prior art has also employed a timer to change the exposure interval. Such timer is, however, provided with only one changeable position, and the provision of two or more switchable positions in the timer is inconvenient from the standpoint of management. If the timer is of the type in which the exposure interval is varied with resistance by which the voltage to the motor is varied, the amount of electrical energy used up by the resistance is not negligible. This results in the disadvantage that the efficiency of the unit will be deleteriously affected.
With the foregoing problems in mind, an object of the present invention is to provide a motor drive unit with the gear ratio made variable so that it is at the normal temperature to effect the operation at or near the B point for the maximum power output, and at a low temperature to effect the operation at or near the A point for the maximum efficiency. Thus, the optimum performance is always assured in any photographic situation which may be encountered.
Another object of the present invention is to provide a combined effect of two kinds of film frequency variation, one which is based on the above-described gear ratio change and the other on the varied time by the timer.